Method and apparatus for transmitting aperiodic channel state information in wireless communication system

ABSTRACT

A method is presented for receiving aperiodic channel state information (CSI). A base station (BS) transmits a CSI request field which is set to trigger a CSI report to a user equipment (UE). The BS receives CSI through a physical uplink shared channel (PUSCH) from the UE. The CSI request field has a value among a plurality of candidate values. The plurality of candidate values comprises a first value which triggers an aperiodic CSI report for a first set of reference signals and a second value which triggers an aperiodic CSI report for a second set of reference signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of co-pending U.S. patent applicationSer. No. 14/983,273 filed on Dec. 29, 2015, which is a Continuation ofU.S. patent application Ser. No. 14/125,051 filed on Dec. 9, 2013 (nowU.S. Pat. No. 9,247,564 issued on Jan. 26, 2016), which is filed as theNational Phase of PCT/KR2012/004516 filed on Jun. 8, 2012, which claimsthe benefit under 35 U.S.C. §119(e) to U.S. Provisional Application No.61/495,396 filed on Jun. 10, 2011, all of which are hereby expresslyincorporated by reference into the present application.

BACKGROUND OF THE INVENTION

The present invention relates to wireless communication and, moreparticularly, to a method and apparatus in which user equipmenttransmits aperiodic channel state information in a wirelesscommunication system.

The data transfer rate over a wireless communication network is recentlyrapidly increasing. This is because a variety of devices, such as smartphones and tablet PCs which require Machine-to-Machine (M2M)communication and a high data transfer rate, are appearing and spread.In order to meet a higher data transfer rate, carrier aggregationtechnology and cognitive radio technology for efficiently using morefrequency bands and multiple antenna technology and multiple basestation cooperation technology for increasing the data capacity within alimited frequency are recently are highlighted.

Furthermore, a wireless communication network is evolving toward atendency that the density of accessible nodes around a user isincreasing. Here, the term ‘node’ may mean antennas or a group ofantennas which are spaced apart from one another in a DistributedAntenna System (DAS). However, the node is not limited to the meaning,but may be used as a broader meaning. That is, the node may become apico eNB (PeNB), a home eNB (HeNB), a Remote Radio Head (RRH), a RemoteRadio Unit (RRU), or a relay. A wireless communication system includingnodes having a high density may have higher system performance throughcooperation between nodes. That is, if the transmission and reception ofeach node are managed by one control station so that the nodes areoperated as antennas or a group of antennas for one cell, the node mayhave much more excellent system performance as compared with when thenodes do not cooperate with each other and thus each node operated as anindependent Base Station (BS) (or an Advanced BS (ABS), a Node-B (NB),an eNode-B (eNB), or an Access Point (AP)). A wireless communicationsystem including a plurality of nodes is hereinafter referred to as amulti-node system.

In a multi-node system, a node which sends a signal to user equipmentmay be different every user equipment, and a plurality of the nodes maybe configured. Here, the nodes may send different reference signals. Thereference signals transmitted by the respective nodes may be transmittedin different subframes because the reference signals have differentsubframe offset values although they have the same transmission cycle.In this case, it may be necessary for a base station to request userequipment to measure a plurality of reference signals transmitted indifferent subframes and feedback the measurement. In the prior art, achannel state information request field is transmitted in each downlinksubframe in which a reference signal, that is, the subject ofmeasurement is transmitted. The conventional method is problematic inthat radio resources are wasted because a channel state informationrequest field must be repeatedly transmitted.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method and apparatusfor transmitting aperiodic channel state information in a wirelesscommunication system.

In an aspect, a method of User Equipment (UE) transmitting aperiodicchannel state information is provided. The method comprising: receivinga channel state information request for a plurality of referencesignals; receiving the plurality of reference signals; generatingchannel state information on each of the plurality of reference signalsin response to the channel state information request; and transmittingthe channel state information through a Physical Uplink Shared Channel(PUSCH), wherein the channel state information request is included inonly some of subframes in which the plurality of reference signals isreceived.

The channel state information request may be included Downlink ControlInformation (DCI) on which the PUSCH is scheduled.

The plurality of reference signals may be placed in a plurality ofdownlink subframes.

The method may further comprises: receiving reference signalconfiguration information on the plurality of reference signals.

The channel state information may be transmitted through a plurality ofthe PUSCHs placed in a plurality of uplink subframes.

The plurality of PUSCHs may be indicated by a plurality of pieces ofscheduling information included in DCI including the channel stateinformation request.

The plurality of PUSCHs may be indicated by a piece of schedulinginformation included in DCI including the channel state informationrequest and information indicating a number of the plurality of PUSCHs.

The channel state information may be generated by measuring referencesignals in a first downlink subframe in which the channel stateinformation request is received and in a second downlink subframe placedprior to the first downlink subframe.

The channel state information may be generated by measuring referencesignals in a first downlink subframe in which the channel stateinformation request is received and in a second downlink subframe placedposterior to the first downlink subframe.

In another aspect, a user equipment (UE) is provided. The UE comprises:a Radio Frequency (RF) unit configured to transmit and receive radiosignals; and a processor coupled to the RF unit, wherein the processorreceives a channel state information request for a plurality ofreference signals, receives the plurality of reference signals,generates channel state information on each of the plurality ofreference signals in response to the channel state information request,and transmits the channel state information through a Physical UplinkShared Channel (PUSCH), wherein the channel state information request isincluded in only some of subframes in which the plurality of referencesignals is received.

The channel state information request may be included Downlink ControlInformation (DCI) on which the PUSCH is scheduled.

The plurality of reference signals may be placed in a plurality ofdownlink subframes.

The processor further receives reference signal configurationinformation on the plurality of reference signals.

The channel state information may be generated by measuring referencesignals in a first downlink subframe in which the channel stateinformation request is received and in a second downlink subframe placedprior to the first downlink subframe.

The channel state information may be generated by measuring referencesignals in a first downlink subframe in which the channel stateinformation request is received and in a second downlink subframe placedposterior to the first downlink subframe.

In a multi-node system, nodes may send different reference signals, anda plurality of nodes may be allocated to single user equipment. If abase station requests aperiodic channel state information feedback, userequipment may measure a plurality of reference signals and feed backaperiodic channel state information. Unlike in the prior art, a basestation may request channel state information feedback for a pluralityof reference signals by sending only a channel state information requestfield once. Accordingly, the waste of radio resources can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a multi-node system.

FIG. 2 shows the structure of a Frequency Division Duplex (FDD) radioframe in 3GPP LTE.

FIG. 3 shows a Time Division Duplex (TDD) radio frame structure in 3GPPLTE.

FIG. 4 illustrates a resource grid for one DL slot.

FIG. 5 shows an example of a DL subframe structure.

FIG. 6 shows the structure of a UL subframe.

FIG. 7 shows an example in which a resource index is mapped to physicalresources.

FIG. 8 shows the mapping of a CRS in a normal cyclic prefix (CP).

FIG. 9 shows the mapping of a CSI-RS for a CSI-RS configuration 0 in anormal CR

FIG. 10 illustrates a plurality of CSI-RSs that should be measured byone UE.

FIG. 11 shows an example in which a plurality of CSI-RSs transmitted inthe same subframe is configured for the same UE.

FIG. 12 shows a method of UE sending CSI according to an embodiment ofthe present invention.

FIG. 13 shows a method of transmitting a CSI request field and feedingback CSI according to an embodiment of the present invention.

FIG. 14 shows another method of transmitting a CSI request field andfeeding back CSI according to an embodiment of the present invention.

FIG. 15 is a block diagram showing a BS and UE.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following technologies may be used in a variety of multiple accessschemes, such as Code Division Multiple Access (CDMA), FrequencyDivision Multiple Access (FDMA), Time Division Multiple Access (TDMA),Orthogonal Frequency Division Multiple Access (OFDMA), and SingleCarrier Frequency Division Multiple Access (SC-FDMA). CDMA may beimplemented using radio technology, such as Universal Terrestrial RadioAccess (UTRA) or CDMA2000. TDMA may be implemented using radiotechnology, such as Global System for Mobile communications(GSM)/General Packet Radio Service (GPRS)/Enhanced Data Rates for GSMEvolution (EDGE). OFDMA may be implemented using radio technology, suchas Institute of Electrical and Electronics Engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, or Evolved UTRA (E-UTRA).IEEE 802.16m is an evolution of IEEE 802.16e, and it provides backwardcompatibility with systems based on IEEE 802.16e. UTRA is part of aUniversal Mobile Telecommunications System (UMTS). 3rd GenerationPartnership Project (3GPP) Long Term Evolution (LTE) is part of anEvolved UMTS (E-UMTS) using E-UTRA, and it adopts OFDMA in downlink andadopts SC-FDMA in uplink. LTE-Advanced (LTE-A) is the evolution of LTE.

FIG. 1 shows an example of a multi-node system.

Referring to FIG. 1, the multi-node system includes a BS and a pluralityof nodes.

In FIG. 1, the node may mean a macro eNB, a pico BS (PeNB), a home eNB(HeNB), a Remote Radio Head (RRH), a relay, or a distributed antenna.The node is also called a point.

In a multi-node system, if the transmission and reception of all nodesare managed by one BS controller and thus each of the nodes is operatedas one cell, this system may be considered as a Distributed AntennaSystem (DAS) which forms one cell. In a DAS, each node may be assignedeach node ID or the nodes may be operated as a set of some antennaswithin a cell without individual node IDs. In other words, a DAS refersto a system in which antennas (i.e., nodes) are distributed and placedat various positions within a cell and the antennas are managed by a BS.The DAS differs from a conventional centralized antenna system (CAS) inwhich the antennas of a BS are concentrated on the center of a cell anddisposed.

In a multi-node system, if each node has each cell ID and performsscheduling and handover, it may be considered as a multi-cell (e.g., amacro cell/femto cell/pico cell) system. If the multi-cells areconfigured in an overlapping manner according to the coverage, this iscalled a multi-tier network.

FIG. 2 shows the structure of a Frequency Division Duplex (FDD) radioframe in 3GPP LTE. This radio frame structure is called a framestructure type 1.

Referring to FIG. 2, the FDD radio frame includes 10 subframes, and onesubframe is defined by two consecutive slots. The time taken for onesubframe to be transmitted is called a Transmission Time Interval (TTI).The time length of a radio frame T_(f)=307200*T_(s)=10 ms and consistsof 20 slots. The time length of one slot T_(slot)=15360*T_(s)=0.5 ms,and the slots are numbered 0 to 19. Downlink (DL) in which each node orBS sends a signal to UE and uplink (UL) in which UE sends a signal toeach node or BS are divided in the frequency region.

FIG. 3 shows a Time Division Duplex (TDD) radio frame structure in 3GPPLTE. This radio frame structure is called a frame structure type 2.

Referring to FIG. 3, the TDD radio frame has a length of 10 ms andconsists of two half-frame each having a length of 5 ms. Furthermore,one half-frame consists of 5 subframes each having a length of 1 ms. Onesubframe is designated as one of a UL subframe, a DL subframe, and aspecial subframe. One radio frame includes at least one UL subframe andat least one DL subframe. One subframe is defined by two consecutiveslots. For example, the length of one subframe may be 1 ms, and thelength of one slot may be 0.5 ms.

The special subframe is a specific period for separating UL and DL fromeach other between a UL subframe and a DL subframe. One radio frameincludes at least one special subframe. The special subframe includes aDownlink Pilot Time Slot (DwPTS), a guard period, and an Uplink PilotTime Slot (UpPTS). The DwPTS is used for initial cell search,synchronization, or channel estimation. The UpPTS is used for channelestimation in a BS and UL transmission synchronization of UE. The guardperiod is a period where interference occurring in UL owing to themulti-path delay of a DL signal is removed between UL and DL.

In the FDD and TDD radio frames, one slot includes a plurality ofOrthogonal Frequency Division Multiplexing (OFDM) symbols in the timedomain and includes a plurality of Resource Blocks (RBs) in thefrequency domain. The OFDM symbol is for representing one symbol periodbecause 3GPP LTE uses OFDMA in DL and may be called another term, suchas an SC-FDMA symbol, according to a multiple access scheme. An RB is aunit of resource allocation and includes a plurality of contiguoussubcarriers in one slot.

The structure of the radio frame is only illustrative, and the number ofsubframes included in the radio frame, the number of slots included inthe subframe, and the number of OFDM symbols included in the slot may bechanged in various ways.

FIG. 4 illustrates a resource grid for one DL slot.

Referring to FIG. 4, one DL slot includes a plurality of OFDM symbols inthe time domain. Here, one DL slot is illustrated as including 7 OFDMAsymbols, and one RB is illustrated as including 12 subcarriers in thefrequency domain, but not limited thereto.

Each element on the resource grid is called a resource element, and oneRB includes 12×7 resource elements. The number N^(DL) of RBs included ina DL slot depends on a DL transmission bandwidth configuration in acell. The resource grid for the DL slot may also be applied to an ULslot.

FIG. 5 shows an example of a DL subframe structure.

Referring to FIG. 5, the subframe includes two contiguous slots. Amaximum of the former 3 OFDM symbols in the first slot of the subframemay correspond to a control region to which DL control channels areallocated, and the remaining OFDM symbols may correspond to a dataregion to which Physical Downlink Shared Channels (PDSCHs) areallocated.

The DL control channel includes a Physical Control Format IndicatorChannel (PCFICH), a Physical Downlink Control Channel (PDCCH), aPhysical Hybrid-ARQ Indicator Channel (PHICH), etc. A PCFICH transmittedin the first OFDM symbol of a subframe carries information about thenumber of OFDM symbols (i.e., the size of a control region) used totransmit control channels within the subframe. Control informationtransmitted through the PDCCH is called Downlink Control Information(DCI). DCI comprises UL resource allocation information, DL resourceallocation information, a UL transmit power control command for specificUE groups, etc. DCI has various formats. A DCI format 0 is used forPUSCH scheduling. Information (field) transmitted through the DCI format0 is as follows.

1) A flag for distinguishing the DCI format 0 and a DCI format 1A (ifthe flag is 0, it indicates the DCI format 0, and if the flag is 1, itindicates the DCI format 1A), 2) A hopping flag (1 bit), 3) RBdesignation and hopping resource allocation, 4) A modulation and codingscheme and redundancy version (5 bits), 5) A new data indicator (I bit),6) A TPC command (2 bits) for a scheduled PUSCH, 7) A cyclic shift (3bits) for a DM-RS, 8) An UL index, 9) a DL designation index (only inTDD), 10) A CQI request, etc. If the number of information bits in theDCI format 0 is smaller than the payload size of the DCI format 1A, ‘0’is padded so that the DCI format 1A is identical with the payload size.

A DCI format 1 is used for one PDSCH codeword scheduling. The DCI format1A is used for the compact scheduling of one PDSCH codeword or a randomaccess process. A DCI format 1B includes precoding information, and itis used for compact scheduling for one PDSCH codeword. A DCI format ICis used for very compact scheduling for one PDSCH codeword. A DCI format1 D includes precoding and power offset information, and it is used forcompact scheduling for one PDSCH codeword. A DCI format 2 is used forPDSCH designation for a closed-loop MIMO operation. A DCI format 2A isused for PDSCH designation for an open-loop MIMO operation. A DCI format3 is used to transmit a TPC command for a PUCCH and a PUSCH throughpower adjustment of 2 bits. A DCI format 3A is used to transmit a TPCcommand for a PUCCH and a PUSCH through power adjustment of 1 bit.

A PHICH carries an Acknowledgement (ACK)/Not-Acknowledgement (NACK)signal for the Hybrid Automatic Repeat Request (HARQ) of UL data. Thatis, an ACK/NACK signal for UL data transmitted by UE is transmitted by aBS on a PHICH.

A PDSCH is a channel on which control information and/or data istransmitted. UE may read data transmitted through a PDSCH by decodingcontrol information transmitted through a PDCCH.

FIG. 6 shows the structure of a UL subframe.

The UL subframe may be divided into a control region and a data regionin the frequency domain. A Physical Uplink Control Channel (PUCCH) onwhich Uplink Control Information (UCI) is transmitted is allocated tothe control region. A Physical Uplink Shared Channel (PUSCH) on which ULdata and/or UL control information is transmitted is allocated to thedata region. In this meaning, the control region may be called a PUCCHregion, and the data region may be called a PUSCH region. UE may supportthe simultaneous transmission of a PUSCH and a PUCCH or may not supportthe simultaneous transmission of a PUSCH and a PUCCH according toconfiguration information indicated by a higher layer.

A PUSCH is mapped to an Uplink Shared Channel (UL-SCH), that is, atransport channel. UL data transmitted on the PUSCH may be a transportblock, that is, a data block for an UL-SCH transmitted for a TTI. Thetransport block may be user information. Alternatively, the UL data maybe multiplexed data. The multiplexed data may include a transport blockand UL control information for an UL-SCH which are multiplexed. Forexample, UL control information multiplexed with UL data may include aChannel Quality Indicator (CQI), a Precoding Matrix Indicator (PMI), aHybrid Automatic Repeat request (HARQ),acknowledgement/not-acknowledgement (ACK/NACK), a Rank Indicator (RI), aPrecoding Type Indication (PTI), and so on. What UL control information,together with UL data, is transmitted in the data region as describedabove is called the piggyback transmission of UCI. Only UL controlinformation may be transmitted in a PUSCH.

A PUCCH for one UE is allocated as a Resource Block (RB) pair in asubframe. Resource blocks belonging to a RB pair occupy differentsubcarriers in a first slot and a second slot. A frequency occupied by aresource block belonging to an RB pair allocated to a PUCCH is changedon the basis of a slot boundary. This is called that the frequency ofthe RB pair allocated to the PUCCH has been frequency-hopped at theboundary of a slot. A frequency diversity gain may be obtained when UEsends UL control information through different subcarriers according toa lapse of time.

A PUCCH carries various types of control information according to aformat. A PUCCH format 1 carries a Scheduling Request (SR). Here, anOn-Off Keying (OOK) scheme may be used. A PUCCH format 1a carries anAcknowledgement/Non-Acknowledgement (ACK/NACK) modulated according to aBinary Phase Shift Keying (BPSK) scheme for one codeword. A PUCCH format1b carries ACK/NACK modulated according to a Quadrature Phase ShiftKeying (QPSK) scheme for two codewords. A PUCCH format 2 carries aChannel Quality Indicator (CQI) modulated according to a QPSK scheme.PUCCH formats 2a and 2b carry a CQI and ACK/NACK. A PUCCH format 3 ismodulated according to a QPSK scheme, and it may carry a plurality ofACK/NACK and SRs.

Each PUCCH format is mapped to a PUCCH region and transmitted. Forexample, the PUCCH formats 2/2a/2b may be mapped to the RB (in FIG. 6,m=0.1) of the edge of a band allocated to UE and then transmitted. Amixed PUCCH RB may be mapped to an RB (e.g., m=2) adjacent in thedirection of the center of the band in the RB to which the PUCCH formats2/2a/2b are allocated and then transmitted. The PUCCH formats 1/1a/1b onwhich an SR and ACK/NACK are transmitted may be disposed in an RB havingm=4 or m=5. UE may be informed of the number N(2)RB of RBs that may beused in the PUCCH formats 2/2a/2b on which a CQI may be transmittedthrough a broadcasted signal.

All PUCCH formats use the Cyclic Shift (CS) of a sequence in each OFDMsymbol. The CS sequence is generated by cyclically shifting a basesequence by a specific CS amount. The specific CS amount is indicated bya CS index.

An example in which the base sequence ru(n) is defined is as follows.

T _(u)(n)=e ^(jb(n)π/4)  [Equation 1]

In Equation 1, u is a root index, n is an element index, 0≦n≦N−1, and Nis the length of the base sequence. b(n) is defined in section 5.5 of3GPP TS 36.211 V8.7.0.

The length of the sequence is equal to the number of elements includedin the sequence. u may be defined by a cell identifier (ID), a slotnumber within a radio frame, etc. Assuming that a base sequence ismapped to one resource block within the frequency domain, the length Nof the base sequence is 12 because one resource block includes 12subcarriers. A different base sequence is defined according to adifferent root index.

A cyclic-shifted sequence r(n, I_(cs)) may be generated by cyclicallyshifting the base sequence r(n) as in Equation 2 below.

$\begin{matrix}{{{r\left( {n,I_{cs}} \right)} = {{r(n)} \cdot {\exp \left( \frac{j\; 2\pi \; I_{cs}n}{N} \right)}}},{0 \leq I_{cs} \leq {N - 1}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

In Equation 2, I_(cs) is a CS index indicating a CS amount(0≦I_(cs)≦N−1).

Available CS indices of the base sequence refer to a CS index that maybe derived from the base sequence according to a CS interval. Forexample, if the length of a base sequence is 12 and a CS interval is 1,a total number of available CS indices of the base sequence is 12. Incontrast, if the length of a base sequence is 12 and a CS interval is 2,a total number of available CS indices of the base sequence is 6. Theorthogonal sequence index i, the CS index I_(cs), and the resource blockindex m are parameters necessary to configure a PUCCH and are resourcesused to distinguish PUCCHs (or UEs) from each other.

In 3GPP LTE, in order for UE to obtain 3 parameters for configuring aPUCCH, resource indices (also called a PUCCH resource index) n⁽¹⁾_(PUCCH), n⁽²⁾ _(PUCCH) are defined. Here, n⁽¹⁾ _(PUCCH) is a resourceindex for the PUCCH formats 1/1a/1b, and n⁽²⁾ _(PUCCH) is a resourceindex for the PUCCH formats 2/2a/2b. A resource index n⁽¹⁾_(PUCCH)=n_(CCE) N⁽¹⁾ _(PUCCH), and n_(CCE) is the number of a first CCEwhich is used to transmit a relevant DCI (i.e., the index of a first CCEwhich is used for relevant PDCCH), and N⁽¹⁾ _(PUCCH) is a parameter thata BS informs UE the parameter through a high layer message. Detailedcontents are as follows.

SPS(semi-persistent scheduled)-UE: defined by RRC Scheduling request:defined by RRC Otherwise: n_(PUCCH) ⁽¹⁾ = n_(CCE) + N_(PUCCH) ⁽¹⁾ (referTS36.213 subclause 10.1[2]) n_(CCE): First CCE(control channel elements)index of PDCCH N_(PUCCH) ⁽¹⁾ = c · N_(sc) ^(RB)/Δ_(shift) ^(PUCCH)$c = \left\{ \begin{matrix}3 & {{normal}\mspace{14mu} {cyclic}{\mspace{11mu} \;}{prefix}} \\2 & {{extended}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}\end{matrix} \right.$ N_(sc) ^(RB) = 12 Δ_(shift) ^(PUCCH) ∈{1, 2, 3}

n⁽²⁾ _(PUCCH) is given a UE-specific way and is semi-staticallyconfigured by a higher layer signal, such as RRC. In LTE, n⁽²⁾ _(PUCCH)is included in an RRC message called ‘CQI-ReportConfig’.

UE determines an orthogonal sequence index, a CS index, etc. using theresource indices n⁽¹⁾ _(PUCCH), n⁽²⁾ _(PUCCH).

UE transmits a PUCCH using physical resources mapped to a resourceindex.

FIG. 7 shows an example in which a resource index is mapped to physicalresources.

UE calculates a resource block index m based on a resource index,allocates physical resources according to a PUCCH format, and transmitthe PUCCH. The following relationship exists between a resource indexallocated to each UE and a mapped physical resource block.

System Parameters Δ_(shift) ^(PUCCH) =1 12 (available cyclic shiftvalue) c = 3 Normal CP N_(PUCCH) ⁽¹⁾ = c · N_(sc) ^(RB)/Δ_(shift)^(PUCCH) = 36 N_(RB) ⁽²⁾ = 2 · N_(sc) ^(RB) = 24 Bandwidth available foruse by PUCCH formats 2/2a/2b (expressed in multiple of N_(sc) ^(RB))N_(cs) ⁽¹⁾ =7 Number of cyclic shifts used for PUCCH formats 1/1a/1b ina resource block with a mix of formats 1/1a/1b and 2/2a/2b

In a multi-node system, a different reference signal may be transmittedfrom each node or each node group. First, a reference signal isdescribed.

In LTE Rel-8, for channel measurement and channel estimation for aPDSCH, a Cell-specific Reference Signal (CRS) is used.

FIG. 8 shows the mapping of a CRS in a normal cyclic prefix (CP).

Referring to FIG. 8, in case of multiple antenna transmission using aplurality of antennas, a resource grid exists in each antenna, and atleast one reference signal for an antenna may be mapped to each resourcegrid. A reference signal for each antenna includes reference symbols. InFIG. 8, Rp indicates the reference symbol of an antenna port p (pε{0, 1,2, 3}). R0 to R3 are not mapped to overlapping resource elements.

In one OFDM symbol, each Rp may be placed at 6 subcarrier intervals. Thenumber of R0s and the number of R1s within a subframe are identical witheach other, and the number of R2s and the number of R3s within asubframe are identical with each other. The number of R2s or R3s withina subframe is smaller than the number of R0s or R1s within the subframe.Rp is not used for any transmission through other antennas other than aNo. p antenna.

In LTE-A, for channel measurement and channel estimation for a PDSCH, aChannel Status Information Reference Signal (CSI-RS) may be usedseparately from a CRS. The CSI-RS is described below.

A CSI-RS, unlike a CRS, includes a maximum of 32 differentconfigurations in order to reduce Inter-Cell Interference (ICI) in amulti-cell environment including heterogeneous network environments.

A configuration for the CSI-RS is different according to the number ofantenna ports within a cell and is given so that maximum differentconfigurations between adjacent cells are configured. The CSI-RS isdivided according to a CP type. The configuration for the CSI-RS isdivided into a configuration applied to both a frame structure type 1and a frame structure type 2 and a configuration applied to only theframe structure type 2 according to a frame structure type (the framestructure type 1 is FDD, and the frame structure type 2 is TDD).

The CSI-RS, unlike the CRS, supports a maximum of 8 antenna ports, andan antenna port p is supported by {15}, {15, 16}, {15, 16, 17, 18}, {15,. . . , 22}. That is, the CSI-RS supports 1, 2, 4, or 8 antenna ports.An interval Δf between subcarriers is defined only for 15 kHz.

A sequence r_(l,ns) (m) for the CSI-RS is generated as in Equationbelow.

$\begin{matrix}{{{{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2m} \right)}}} \right)} + {j\frac{1}{\sqrt{2}}\left( {1 - {c \cdot \left( {{2m} + 1} \right)}} \right)}}},\mspace{20mu} {m = 0},\ldots \mspace{14mu},{N_{RB}^{\max,\; {DL}} - {1\mspace{31mu} {where}}},{c_{init} = {{2^{10} \cdot \left( {{7 \cdot \left( {n_{s} + 1} \right)} + l + 1} \right) \cdot \left( {{2 \cdot N_{ID}^{cell}} + 1} \right)} + {2 \cdot N_{ID}^{cell}} + N_{{CP}}}}}\mspace{20mu} {N_{CP} = \left\{ \begin{matrix}1 & {{for}\mspace{14mu} {normal}\mspace{14mu} {CP}} \\0 & {{for}\mspace{14mu} {extended}\mspace{14mu} {CP}}\end{matrix} \right.}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

In Equation 3, ns is a slot number within a radio frame, and 1 is anOFDM symbol number within the slot. c(i) is a pseudo random sequence andis started from each OFDM symbol as c_(init)·N_(ID) ^(cell) indicates aphysical layer cell ID.

In subframes configured to transmit a CSI-RS, a reference signalsequence r_(l,ns)(m) is mapped to a complex value modulation symbola_(k,l)(p) used as a reference symbol for an antenna port p.

A relationship between r_(l,ns)(m) and a_(k,l)(p) is defined as inEquation below.

$\begin{matrix}{\mspace{79mu} {{a_{k,l}^{(p)} = {{w_{I^{''}} \cdot {r(m)}}\mspace{31mu} {where}}},\; {k = {k^{\prime} + {12m} + \left\{ {{\begin{matrix}{- 0} & {{{{for}\mspace{14mu} p} \in \left\{ {15,16} \right\}},{{normal}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}} \\{- 6} & {{{{for}\mspace{14mu} p} \in \left\{ {17,18} \right\}},{{normal}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}} \\{- 1} & {{{{for}\mspace{14mu} p} \in \left\{ {19,20} \right\}},{{normal}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}} \\{- 7} & {{{{for}\mspace{14mu} p} \in \left\{ {21,22} \right\}},{{normal}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}} \\{- 0} & {{{{for}\mspace{14mu} p} \in \left\{ {15,16} \right\}},{{extended}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}} \\{- 3} & {{{{for}\mspace{14mu} p} \in \left\{ {17,18} \right\}},{{extended}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}} \\{- 6} & {{{{for}\mspace{14mu} p} \in \left\{ {19,20} \right\}},{{extended}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}} \\{- 9} & {{{{for}\mspace{14mu} p} \in \left\{ {21,22} \right\}},{{extended}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}}\end{matrix}l} = {l^{\prime} + \left\{ {{\begin{matrix}l^{''} & \begin{matrix}{{{CSI}\mspace{14mu} {reference}\mspace{14mu} {signal}\mspace{14mu} {configurations}\mspace{14mu} 0\text{-}19},} \\{{normal}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}\end{matrix} \\{2l^{''}} & \begin{matrix}{{{CSI}\mspace{14mu} {reference}\mspace{14mu} {signal}\mspace{14mu} {configurations}\mspace{14mu} 20\text{-}31},} \\{{normal}\mspace{14mu} {cyclic}\mspace{14mu} {prefix}}\end{matrix} \\l^{''} & \begin{matrix}{{{CSI}\mspace{14mu} {reference}\mspace{14mu} {signal}\mspace{14mu} {configurations}\mspace{14mu} 0\text{-}27},} \\{{extended}\mspace{20mu} {cyclic}\mspace{14mu} {prefix}}\end{matrix}\end{matrix}\mspace{20mu} w_{l^{''}}} = \left\{ {{{\begin{matrix}1 & {p \in \left\{ {15,17,19,21} \right\}} \\\left( {1 -} \right)^{l^{''}} & {p \in \left\{ {16,18,20,22} \right\}}\end{matrix}\mspace{20mu} l^{''}} = 0},{{1\mspace{20mu} m} = 0},1,\ldots \mspace{14mu},{{N_{RB}^{DL} - {1\mspace{20mu} m^{\prime}}} = {m + \left\lfloor \frac{N_{RB}^{\max,{DL}} - N_{RB}^{DL}}{2} \right\rfloor}}} \right.} \right.}} \right.}}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

In Equation 4, (k′, l′) and ns are given in Table 1 and Table 2 below. ACSI-RS may be transmitted in a DL slot in which (ns mod 2) meets theconditions of Table 1 and Table 2 (mod means a modular operation, thatis, mod means the remainder obtained by dividing ns by 2).

Table below shows a CSI-RS configuration for a normal CP.

TABLE 1 Number oF CSI reference signals configured CSI reference signal1 or 2 4 8 configuration (k′, l′) n_(s) mod 2 (k′, l′) n_(s) mod 2 (k′,l′) n_(s) mod 2 Frame structure 0 (9, 5) 0 (9, 5) 0 (9, 5) 0 type 1 1(11, 2)  1 (11, 2)  1 (11, 2)  1 and 2 2 (9, 2) 1 (9, 2) 1 (9, 2) 1 3(7, 2) 1 (7, 2) 1 (7, 2) 1 4 (9, 5) 1 (9, 5) 1 (9, 5) 1 5 (8, 5) 0 (8,5) 0 6 (10, 2)  1 (10, 2)  1 7 (8, 2) 1 (8, 2) 1 8 (6, 2) 1 (6, 2) 1 9(8, 5) 1 (8, 5) 1 10 (3, 5) 0 11 (2, 5) 0 12 (5, 2) 1 13 (4, 2) 1 14 (3,2) 1 15 (2, 2) 1 16 (1, 2) 1 17 (0, 2) 1 18 (3, 5) 1 19 (2, 5) 1 Framestructure 20 (11, 1)  1 (11, 1)  1 (11, 1)  1 type 2 only 21 (9, 1) 1(9, 1) 1 (9, 1) 1 22 (7, 1) 1 (7, 1) 1 (7, 1) 1 23 (10, 1)  1 (10, 1)  124 (8, 1) 1 (8, 1) 1 25 (6, 1) 1 (6, 1) 1 26 (5, 1) 1 27 (4, 1) 1 28(3, 1) 1 29 (2, 1) 1 30 (1, 1) 1 31 (0, 1) 1

Table below shows a CSI-RS configuration for an extended CP.

TABLE 2 Number of CSI reference signals configured CSI reference signal1 or 2 4 8 configuration (k′, l′) n_(s) mode 2 (k′, l′) n_(s) mode 2(k′, l′) n_(s) mode 2 Frame structure 0 (11, 4)  0 (11, 4)  0 (11, 4)  0type 1 and 2 1 (9, 4) 0 (9, 4) 0 (9, 4) 0 2 (10, 4)  1 (10, 4)  1 (10,4)  1 3 (9, 4) 1 (9, 4) 1 (9, 4) 1 4 (5, 4) 0 (5, 4) 0 5 (3, 4) 0 (3, 4)0 6 (4, 4) 1 (4, 4) 1 7 (3, 4) 1 (3, 4) 1 8 (8, 4) 0 9 (6, 4) 0 10 (2,4) 0 11 (0, 4) 0 12 (7, 4) 1 13 (6, 4) 1 14 (1, 4) 1 15 (0, 4) 1 Framestructure 16 (11, 1)  1 (11, 1)  1 (11, 1)  1 type 2 only 17 (10, 1)  1(10, 1)  1 (10, 1)  1 18 (9, 1) 1 (9, 1) 1 (9, 1) 1 19 (5, 1) 1 (5, 1) 120 (4, 1) 1 (4, 1) 1 21 (3, 1) 1 (3, 1) 1 22 (8, 1) 1 23 (7, 1) 1 24(6, 1) 1 25 (2, 1) 1 26 (1, 1) 1 27 (0, 1) 1

A subframe including a CSI-RS must satisfy Equation below.

(10n _(f) +└n _(s)/2┘Δ_(CSI-RS))mod T _(CSI-RS)=0  [Equation 5]

Furthermore, the CSI-RS may be transmitted in a subframe satisfying thecondition of Table 3.

Table 3 shows a CSI-RS subframe configuration related to a duty cycle.n_(f) is a system frame number.

TABLE 3 CSI-RS periodicity CSI-RS subframe offset CSI-RS-SubframeConfigT_(CSI-RS) Δ_(CSI-RS) I_(CSI-RS) (subframes) (subframes) 0-4 5I_(CSI-RS)  5-14 10 I_(CSI-RS) - 5 15-34 20 I_(CSI-RS) - 15 35-74 40I_(CSI-RS) - 35  75-154 80 I_(CSI-RS) - 75

In Table 3, ‘CSI-RS-SubframeConfig’, that is, I_(CSI-RS) is a valuegiven by a higher layer, and it indicates a CSI-RS subframeconfiguration. T_(CSI-RS) indicates a cell-specific subframeconfiguration period, and Δ_(CSI-RS) indicates a cell-specific subframeoffset. A CSI-RS supports five types of duty cycles according to aCQI/CSI feedback, and it may be transmitted with a different subframeoffset in each cell.

FIG. 9 shows the mapping of a CSI-RS for a CSI-RS configuration 0 in anormal CP.

Referring to FIG. 9, two antenna ports transmit a CSI-RS using, forexample, two same contiguous resource elements for p={15, 16}, {17, 18},{19, 20}, {21, 22}, but using an Orthogonal Cover Code (OCC).

A plurality of CSI-RS configurations can be used in a cell. In thiscase, one CSI-RS configuration in which UE assumes non-zero transmitpower and one or non CSI-RS configuration in which UE assumes zerotransmit power may be configured.

A CSI-RS is not transmitted in the following cases.

1. A special subframe of the frame structure type 2.

2. When it is collided with a synchronization signal, a physicalbroadcast channel (PBCH), or a system information block (SIB).

3. A subframe in which a paging message is transmitted.

A resource element (k,l) used to transmit a CSI-RS for a specificantenna port of a set S is not used to transmit a PDSCH for a specificantenna port in the same slot. Furthermore, the resource element (k,l)is not used to transmit a CSI-RS for another specific antenna port otherthan the set S in the same slot. Here, antenna ports included in the setS include {15, 16}, {17, 18}, {19, 20}, and {21, 22}.

Parameters necessary to transmit the CSI-RS include 1. a CSI-RS portnumber, 2. CSI-RS configuration information, 3. a CSI-RS subframeconfiguration I_(CSI-RS), 4. a subframe configuration periodicityT_(CSI-RS), 5. a subframe offset A_(CSI-RS), and so on. The parametersare cell-specific and are given through higher layer signaling.

A BS may apply a reference signal, such as a CRS and a CSI-RS, so thatUE may identify each node in a multi-node system.

UE may measure the reference signal, generate Channel State Information(CSI), and then report or feed back the CSI to a BS or a node. CSIincludes a CQI, a PMI, an RI, etc.

A method of sending Channel State Information (CSI) includes a periodictransmission method and an aperiodic transmission method.

In the periodic transmission method, CSI may be transmitted through aPUCCH or a PUSCH. The aperiodic transmission method is performed in sucha manner that, if more precise CSI is necessary, a BS requests CSI fromUE. A BS sends a CSI request (e.g., a CQI request included in the DCIformat 0), and UE measures the reference signal of a subframe includingthe CSI request and feeds back CSI. The aperiodic transmission method isperformed through a PUSCH. Since a PUSCH is used, capacity is greaterand detailed channel state reporting possible. If periodic transmissionand aperiodic transmission collide with each other, only aperiodictransmission is performed.

An aperiodic CSI feedback is performed when there is a request from aBS. If UE is accessed, a BS may request a CSI feedback from the UE whensending a random access response grant to the UE. In some embodiments, aBS may request a CSI feedback from UE by using a DCI format in which ULscheduling information is transmitted. A CSI request field requesting aCSI feedback comprises 1 bit or 2 bits. If the CSI request field is 1bit, in case of ‘0’, a CSI report is not triggered. In case of ‘1’, aCSI report is triggered. In case of 2 bits, the following Table isapplied.

TABLE 4 Value of CSI request field Description ‘00’ No aperiodic CSIreport is triggered ‘01’ Aperiodic CSI report triggered for servingcell^(c) ‘10’ Aperiodic CSI report is triggered for a 1^(st) set ofserving cells configured by higher layers ‘11’ Aperiodic CSI report istriggered for a 2^(nd) set of serving cells configured by higher layers

When a CSI report is triggered by a CSI request field, UE feeds back CSIthrough PUSCH resources designated in the DCI format 0. Here, what CSIwill be fed back is determined according to a reporting mode. Forexample, which one of a wideband CQI, a UE-selective CQI, and a higherlayer configuration CQI will be fed back is determined according to areporting mode. Furthermore, what kind of a PMI will be fed back is alsodetermined along with a CQI. A PUSCH reporting mode is semi-staticallyconfigured through a higher layer message, and an example thereof islisted in Table 5 below.

TABLE 5 PMI Feedback Type Single Multiple No PMI PMI PMI PUSCH CQIWideband Mode 1-2 Feedback Type (wideband CQI) UE Selected Mode 2-0 Mode2-2 (subband CQI) Higher Layer- Mode 3-0 Mode 3-1 configured (subbandCQI)

Unlike aperiodic CSI feedback transmitted only when it is triggeredthrough a PDCCH, periodic CSI feedback is semi-statically configuredthrough a higher layer message. The periodicity N_(pd) and subframeoffset N_(OFFSET,CQI) of periodic CSI feedback are transferred to UE asa higher layer message (e.g., an RRC message) through a parameter called‘cqi-pmi-ConfigIndex’ (i.e., I_(CQI/PMI)). A relationship between theparameter I_(CQI/PMI) and the periodicity and subframe offset is listedin Table 6 in case of FDD and in Table 7 in case of TDD.

TABLE 6 Value of I_(CQI/PMI) Value of N_(pd) N_(OFFSET,CQI) 0 ≦I_(CQI/PMI) ≦ 1 2 I_(CQI/PMI) 2 ≦ I_(CQI/PMI) ≦ 6 5 I_(CQI/PMI) - 2 7 ≦I_(CQI/PMI) ≦ 16 10 I_(CQI/PMI) - 7 17 ≦ I_(CQI/PMI) ≦ 36 20I_(CQI/PMI) - 17 37 ≦ I_(CQI/PMI) ≦ 76 40 I_(CQI/PMI) - 37 77 ≦I_(CQI/PMI) ≦ 156 80 I_(CQI/PMI) - 77 157 ≦ I_(CQI/PMI) ≦ 316 160I_(CQI/PMI) - 157 I_(CQI/PMI) = 317 Reserved 318 ≦ I_(CQI/PMI) ≦ 349 32I_(CQI/PMI) - 318 350 ≦ I_(CQI/PMI) ≦ 413 64 I_(CQI/PMI) - 350 414 ≦I_(CQI/PMI) ≦ 541 128 I_(CQI/PMI) - 414 542 ≦ I_(CQI/PMI) ≦ 1023Reserved

TABLE 7 Value of I_(CQI/PMI) Value of N_(pd) N_(OFFSET,CQI) I_(CQI/PMI)= 0 1 I_(CQI/PMI) 1 ≦ I_(CQI/PMI) ≦ 5 5 I_(CQI/PMI) - 1 6 ≦ I_(CQI/PMI)≦ 15 10 I_(CQI/PMI) - 6 16 ≦ I_(CQI/PMI) ≦ 35 20 I_(CQI/PMI) - 16 36 ≦I_(CQI/PMI) ≦ 75 40 I_(CQI/PMI) - 36 76 ≦ I_(CQI/PMI) ≦ 155 80I_(CQI/PMI) - 76 156 ≦ I_(CQI/PMI) ≦ 315 160 I_(CQI/PMI) - 156 316 ≦I_(CQI/PMI) ≦ 1023 Reserved

A periodic PUCCH reporting mode is listed in Table below.

TABLE 8 PMI Feedback Type Single No PMI PMI PUCCH CQI Wideband Mode 1-0Mode 1-1 Feedback Type (wideband CQI) UE Selected Mode 2-0 Mode 2-1(subband CQI)

UE must measure the reference signal of a specific resource region inorder to feed back CSI, for example, CQI. Resources that must bemeasured in order to generated CQI are called CQI reference resources.It is assumed that UE feeds back CQI in a UL subframe n. Here, a CQIreference resource is defined as a group of DL physical resource blockscorresponding to a frequency band which is related to a CQI value in thefrequency domain and is defined as one DL subframe n-n_(CQI) _(_) _(ref)in the time domain.

In periodic CQI feedback, n_(CQI) _(_) _(ref) is the smallest value fromamong 4 or more values corresponding to a valid DL subframe. Inaperiodic CQI feedback, n_(CQI) _(_) _(ref) indicates a valid DLsubframe including a UL DCI format including a relevant CQI request.That is, CQI reference resource is a valid DL subframe including a CQIrequest filed in aperiodic CQI feedback.

In aperiodic CQI feedback, if the DL subframe n-n_(CQI) _(_) _(ref) isreceived after a subframe including a CQI request included in a randomaccess response grant, n_(CQI) _(_) _(ref) is 4, and the DL subframen-n_(CQI) _(_) _(ref) corresponds to a valid DL subframe.

A DL subframe is considered as a valid DL subframe to a UE if it meetsthe following conditions.

1) The DL subframe is configured for the UE, 2) Except for transmissionmode 9, the DL subframe is not a Multicast-Broadcast Single FrequencyNetwork (MBSFN) subframe, 3) the DL subframe does not contain a DwPTSfield in case the length of DwPTS field is 7680 Ts and less (here,307200 Ts=10 ms), and 4) the DL subframe should not correspond to aconfigured measurement gap for the UE.

If a valid DL subframe for CQI reference resources does not exist, CQIfeedback is omitted in UL subframe n.

In the layer domain, CQI reference resources are defined by any RI andPMI value on which the CQI is conditioned.

In CQI reference resources, UE is operated under the followingassumption in order to derive a CQI index.

1. In CQI reference resources, the first 3 OFDM symbols are occupied bya control signal.

2. In CQI reference resources, there is no resource element used by aPrimary Synchronization Signal (PSS), a Secondary Synchronization Signal(SSS), or a Physical Broadcast Channel (PBCH).

3. In CQI reference resources, the CP length of a non-MBSFN subframe isassumed.

4. Redundancy version 0

Table below shows the transmission modes of a PDSCH assumed for CQIreference resources.

TABLE 9 Transmission mode Transmission scheme of PDSCH 1 Single-antennaport, port 0 2 Transmit diversity 3 Transmit diversity if the associatedrank indicator is 1, otherwise large delay CDD 4 Closed-loop spatialmultiplexing 5 Multi-user MIMO 6 Closed-loop spatial multiplexing with asingle transmission layer 7 If the number of PBCH antenna ports is one,Single-antenna port, port 0; otherwise Transmit diversity 8 If the UE isconfigured without PMI/RI reporting: if the number of PBCH antenna portsis one, single-antenna port, port 0; otherwise transmit diversity If theUE is configured with PMI/RI reporting: closed-loop spatial multiplexing9 Closed-loop spatial multiplexing with up to 8 layer transmission,ports 7-14

In the transmission mode 9 and a feedback (reporting) mode thereof, UEperforms channel measurement for calculating CQI based on only a CSI-RS.In other transmission modes and relevant reporting modes, UE performschannel measurement for calculating CQI based on a Cell-specific RS(CRS). UE reports the highest CQI index value of CQI indices 1 to 15shown in Table below under a specific condition. The specific conditionincludes a modulation scheme corresponding to a CQI index and that asingle PDSCH transport block having a transport block size must bereceived within a 0.1 error probability when the single PDSCH transportblock occupies CQI reference resources.

A CQI index fed back by UE and its meanings are listed in Table below.

TABLE 10 CQI index modulation code rate × 1024 efficiency 0 out of range1 QPSK 78 0.1523 2 QPSK 120 0.2344 3 QPSK 193 0.3770 4 QPSK 308 0.6016 5QPSK 449 0.8770 6 QPSK 602 1.1758 7 16QAM 378 1.4766 8 16QAM 490 1.91419 16QAM 616 2.4063 10 64QAM 466 2.7305 11 64QAM 567 3.3223 12 64QAM 6663.9023 13 64QAM 772 4.5234 14 64QAM 873 5.1152 15 64QAM 948 5.5547

In a multi-node system, a plurality of nodes or a node group may beallocated to UE, and different reference signals may be used inrespective nodes or a node group. In this case, a BS may requestaperiodic CSI feedback (reporting) for a plurality of reference signalsfrom UE. In response to the request, the UE may measure the plurality ofreference signals and report CSI (e.g., a CQI) on each of the referencesignals.

FIG. 10 illustrates a plurality of CSI-RSs that should be measured byone UE.

Referring to FIG. 10, a CSI-RS #0 and a CSI-RS #1 may be configured forUE. The CSI-RS #0 may be a CSI-RS transmitted by a node #N, and theCSI-RS #1 may be a CSI-RS transmitted by a node #M.

The transmission periodicity of the CSI-RS #0 may be identical with thetransmission periodicity of the CSI-RS #1. For example, the CSI-RS #0may be transmitted in a subframe n+10 m (m is 0 or a natural number).The CSI-RS #1 may be transmitted in a subframe n+1+10 m.

As shown in FIG. 10, CSI-RSs transmitted in different subframes may beconfigured for the same UE, but not limited thereto. That is, aplurality of CSI-RSs transmitted in the same subframe may be configuredfor the same UE.

FIG. 11 shows an example in which a plurality of CSI-RSs transmitted inthe same subframe is configured for the same UE.

Referring to FIG. 11, CSI-RS #0 and #1 are transmitted in a subframe n.The CSI-RS #0 may be a CSI-RS transmitted by a node #N, and the CSI-RS#1 may be a CSI-RS transmitted by a node #M.

As described above, a plurality of CSI-RSs may be configured for thesame UE. Here, if a BS requests aperiodic CSI from the UE, the UE mustsend a CSI request field (e.g., a CQI request field) in each subframe inwhich the reference signal is transmitted, in a conventional method. If,as in a multiple node system, a plurality of reference signals istransmitted to UE in different subframes, inefficiency may occur andresources may be wasted in a conventional method because a CSI requestfield must be repeatedly transmitted in each subframe.

In order to solve the problems, the present invention provides a methodof performing aperiodic CSI feedback for a plurality of referencesignals in such a manner that a BS triggers CSI feedback by sending aCSI request field to UE once.

FIG. 12 shows a method of UE sending CSI according to an embodiment ofthe present invention.

Referring to FIG. 12, the UE receives reference signal configurationinformation corresponding to each node at step S110.

The reference signal configuration information may be received through ahigher layer signal, such as an RRC message, and it may inform that whatreference signal is transmitted by each node. For example, the referencesignal configuration information may inform the configuration of aCSI-RS transmitted by each node.

The UE receives a CSI request field at step S120. The CSI request fieldtriggers aperiodic CSI reporting for the UE. The existing CSI requestfield includes a request for CSI feedback for a specific cell or aspecific carrier. In contrast, the CSI request field according to thepresent invention may include a request for CSI feedback for a pluralityof reference signals.

Table below shows an example of the CSI request field.

TABLE 11 Value of CSI request field Description ‘000’ No aperiodic CSIreport is triggered ‘001’ Aperiodic CSI report triggered for servingcell^(c) ‘010’ Aperiodic CSI report is triggered for a 1^(st) set ofserving cells configured by higher layers ‘011’ Aperiodic CSI report istriggered for a 2^(nd) set of serving cells configured by higher layers‘100’ Aperiodic CSI report is triggered for a 1^(nd) set of referencesignals or antenna ports configured by higher layers ‘101’ Aperiodic CSIreport is triggered for a 2^(nd) set of reference signals or antennaports configured by higher layers

As shown in Table 11, if a value of the CSI request field is ‘100’ or‘101’, a CSI report on a first or second set of reference signals may betriggered. The first set or the second set may denote a set of referencesignals configured by a higher layer signal, and the reference signalmay be a CSI-RS transmitted by each node.

For example, the first set may be a set of a plurality of referencesignals transmitted in different subframes, as in the CSI-RSs #0 and #1illustrated in FIG. 10. Furthermore, the second set may be a set of aplurality of reference signals transmitted in the same subframe, as inthe CSI-RSs #0 and #1 illustrated in FIG. 11. However, Table 11 is onlyillustrative, and the first or second set may denote a combination ofother reference signals or a combination of other nodes.

For example, a CSI request field may include 1. A request for CSI whenonly some of antenna ports in which a specific CSI-RS is transmittedparticipate in PDSCH transmission or 2. A request for CSI when allantenna ports in which a CSI-RS is transmitted participate in PDSCHtransmission.

From a viewpoint of UE, a CSI request field may be included in DCI andreceived through PDCCHs. The DCI including the CSI request field may bepieces of DCI for scheduling PUSCHs, such as the DCI format 0 and theDCI format 4.

In some embodiments, the CSI request field may be received through ahigher layer signal, such as an RRC message.

The UE measures a plurality of reference signals in response to the CSIrequest field and generates CSI on each of the reference signals at stepS130. The CSI may be a CQI, but not limited, and it is evident that theCSI may include a Rank Indicator (RI), a Precoding Matrix Indicator(PMI), etc.

The UE sends the CSI on each of the reference signals through PUSCHresources at step S140. The PUSCH resources may exist within onesubframe or may exist within a plurality of subframes.

A process in which UE generates CSI in response to a CSI request fieldand then sends the CSI through PUSCH resources is described in detailbelow.

In the present invention, a CSI request field is not transmitted in allsubframes whose reference signals must be measured in order to generateCSI. That is, in the prior art, if a CSI request field is included in aDCI format including UL scheduling information, UE measures a referencesignal in a valid DL subframe in which the CSI request field has beenreceived and generates CSI based on the measurement. In contrast, in thepresent invention, if reference signals that must be measured by UE areplaced in a plurality of subframes, a CSI request field may betransmitted in only some of the plurality of subframes.

UE may know that CSI on what reference signal must be generated based ona value of a CSI request field and also know the transmission cycle,subframe offset, pattern, etc. of each reference signal through a higherlayer signal, such as an RRC message. Accordingly, UE may know theposition of a reference signal (i.e., CSI reference resources), that is,the subject of measurement through a CSI request field and a higherlayer signal.

FIG. 13 shows a method of transmitting a CSI request field and feedingback CSI according to an embodiment of the present invention. It isassumed that UE is requested to report aperiodic CSI on CSI-RSs whichare transmitted in subframes n and n+1.

Referring to FIG. 13, a BS sends a CSI request field in the subframe nthrough DCI including PUSCH scheduling information. Furthermore, the BSsends the CSI-RSs in the subframe n and the subframe n+1.

In this case, UE analyzes CSI reference resources up to the subframe n+1without being limited to the subframe n. That is, the UE includes thevalid DL subframe n+1, placed posterior to the subframe n including theCSI request field, in the CSI reference resources.

FIG. 14 shows another method of transmitting a CSI request field andfeeding back CSI according to an embodiment of the present invention. Itis assumed that UE is requested to report aperiodic CSI on CSI-RSs whichare transmitted in subframes n and n+1.

Referring to FIG. 14, a BS sends a CSI request field through DCI,including PUSCH scheduling information, in the subframe n+1. The UEincludes the valid DL subframe n, placed anterior to the subframe n+1including the CSI request field, in the CSI reference resources.

That is, in the prior art, CSI on only a subframe in which a CSI requestfield is transmitted is measured. In contrast, in the present invention,CSI is measured with consideration taken of a subframe in which a CSIrequest field is not transmitted, and the measured CSI is reported.

FIGS. 13 and 14 show examples in which UE sends CSI on CSI-RSs receivedin a plurality of subframes through a single subframe, but not limitedthereto.

That is, UE may send CSI through a plurality of subframes. In this case,DCI including a CSI request field may include pieces of PUSCH schedulinginformation for scheduling a plurality of PUSCH resources.

In some embodiments, a plurality of PUSCH resources may be previouslydefined so that they are consecutively allocated physically orlogically. In this case, a piece of PUSCH scheduling information and thenumber of allocated PUSCHs may be informed so that the plurality ofPUSCH resources can be scheduled.

In the present invention, the multiple node system has been described asan example in order to help understanding of contents, but not limitedthereto. That is, the present invention may be applied to any system inwhich multiple CSI-RSs are configured. Furthermore, a CQI has beenchiefly described as an example of CSI, but an RI, a PMI, etc. maybecome an example of CSI.

FIG. 15 is a block diagram showing a BS and UE.

The BS 100 includes a processor 110, memory 120, and a Radio Frequency(RF) unit 130. The processor 110 implements the proposed functions,processes, and methods. For example, the processor 110 may sendreference signal configuration information, informing the configurationof reference signals allocated to each node, to UE. Furthermore, theprocessor 110 may send a CSI request field, but the CSI request field istransmitted in only some of subframes in which a plurality of referencesignals is transmitted. The memory 120 is coupled to the processor 110and is configured to store various pieces of information for driving theprocessor 110. The RF unit 130 is coupled to the processor 110 and isconfigured to send and/or receive radio signals. The RF unit 130 may beformed of a plurality of nodes coupled to the BS 100 in a wired manner.

The UE 200 includes a processor 210, memory 220, and an RF unit 230. Theprocessor 210 performs the above-described functions and methods. Forexample, the processor 210 may receive reference signal configurationinformation and a CSI request field from a BS or a node. The CSI requestfield may be included in DCI or received through a higher layer signal,such as an RRC message. The UE generates CSI on a plurality of referencesignals (e.g., CSI-RSs transmitted by respective nodes) based on a valueof a CSI request field and sends the CSI. In this case, the CSI-RSs maybe received in a plurality of subframes or may be received in a singlesubframe. The generated CSI may be transmitted through a single PUSCH ora plurality of PUSCHs. The memory 220 is coupled to the processor 210and is configured to store various pieces of information for driving theprocessor 210. The RF unit 230 is coupled to the processor 210 and isconfigured to send and/or receive radio signals.

The processor 110, 210 may include Application-Specific IntegratedCircuits (ASICs), other chipsets, logic circuits, or data processorsand/or converters for mutually converting baseband signals and radiosignals. The memory 120, 220 may include Read-Only Memory (ROM), RandomAccess Memory (RAM), flash memory, memory cards, storage media and/orother storage devices. The RF unit 130, 230 may include one or moreantennas for transmitting and/or receiving radio signals. When anembodiment is implemented in software, the above-described scheme may beimplemented using a module (process or function) that performs the abovefunction. The module may be stored in the memory 120, 220 and executedby the processor 110, 210. The memory 120, 220 may be placed inside oroutside the processor 110, 210 and connected to the processor 110, 210using a variety of well-known means.

The present invention may be implemented using hardware, software, or acombination of them. In hardware implementations, the present inventionmay be implemented using Application Specific Integrated Circuits(ASICs), Digital Signal Processors (DSPs), Programmable Logic Devices(PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers,microprocessors, other electronic units, or a combination of them, whichare designed to perform the above function. In software implementations,the present invention may be implemented using a module performing theabove function. The software may be stored in the memory and executed bythe processor. The memory or the processor may adopt various means wellknown to those skilled in the art.

Although the preferred embodiments of the present invention have beendescribed in detail, a person having ordinary skill in the art willappreciate that the present invention may be modified in various wayswithout departing from the spirit and scope of the present inventiondefined in the appended claims. Accordingly, a change of futureembodiments of the present invention may not deviate from the technologyof the present invention.

1. (canceled)
 2. A method for transmitting aperiodic channel state information (CSI), the method performed by a user equipment (UE) and comprising, receiving an uplink downlink control information (DCI) format; and transmitting aperiodic CSI through a Physical Uplink Shared Channel (PUSCH) if a CSI request field included in the uplink DCI format is set to trigger an aperiodic CSI report, wherein the CSI request field consists of 1 bit or a plurality of bits, and wherein, when the UE is configured with more than one channel state information-reference signal (CSI-RS), the CSI request field consisting of the plurality of bits applies to the UE.
 3. The method of claim 2, wherein the CSI request field has a value among a plurality of candidate values, wherein the plurality of candidate values comprises a first value which triggers an aperiodic CSI report for a first set of reference signals and a second value which triggers an aperiodic CSI report for a second set of reference signals, and wherein the first set of reference signals and the second set of reference signals are configured by a higher layer signal.
 4. The method of claim 2, wherein the PUSCH is scheduled by the uplink DCI.
 5. The method of claim 2, wherein the plurality of bits are 2 bits.
 6. The method of claim 3, wherein the higher layer signal is a radio resource control (RRC) message.
 7. A user equipment (UE), comprising: a Radio Frequency (RF) unit configured to transmit and receive radio signals; and a processor coupled to the RF unit, wherein the processor is configured to: control the RF unit to receive an uplink downlink control information (DCI) format and control the RF unit to transmit aperiodic channel state information (CSI) through a Physical Uplink Shared Channel (PUSCH) if a CSI request field included in the uplink DCI format is set to trigger an aperiodic CSI report, wherein the CSI request field consists of 1 bit or a plurality of bits, and wherein, when the UE is configured with more than one channel state information-reference signal (CSI-RS), the CSI request field consisting of the plurality of bits applies to the UE.
 8. The UE of claim 7, wherein the CSI request field has a value among a plurality of candidate values, wherein the plurality of candidate values comprises a first value which triggers an aperiodic CSI report for a first set of reference signals and a second value which triggers an aperiodic CSI report for a second set of reference signals, and wherein the first set of reference signals and the second set of reference signals are configured by a higher layer signal.
 9. The UE of claim 7, wherein the PUSCH is scheduled by the uplink DCI.
 10. The UE of claim 7, wherein the plurality of bits are 2 bits.
 11. The UE of claim 8, wherein the higher layer signal is a radio resource control (RRC) message. 